Zev, a couple ideas come to mind. You said you tried larger pump jets, but it sounds like you are not getting enough accelerator pump fuel delivered to the intake stream when it transitions off of the idle circuit. With 45MM carbs with large ventures. When you open the throttle the air velocity slows way down, and the fuel will drop out of the air steam. You need a good shot of fuel to overcome this. Double check that the accelerator jets are squirting forward and at the same velocity.
I had the same stumbling issue and two of the accelerator jets were clogged with debris on my carbs. The only way I found this was taking the carb of and watching the fuel come out of the accelerator pump jets. It drove me crazy until I found this issue.
If you have a wideband controller installer you could see this lean condition when you open the throttle and transition off the idle circuit to the main fuel circuit.
If you PM me. I can share my jetting chart that has worked for my 45mm Solexes. At least you have some more data as jets are expense.
Hope this helps.